1. Field of the Invention
The present invention relates to light sources, and more specifically, to a hybrid light source having a high-efficiency lamp, a low-efficiency lamp, and drive circuits for controlling the amount of power delivered to each of the lamps.
2. Description of the Related Art
In order to reduce energy consumption, the use of high-efficiency light sources (e.g., high-efficacy light sources) is increasing, while the use of low-efficiency light sources (e.g., low-efficacy light sources) is decreasing. High-efficiency light sources may comprise high-efficacy lamps, for example, gas discharge lamps (such as compact fluorescent lamps), phosphor-based lamps, high-intensity discharge (HID) lamps, and light-emitting diode (LED) light sources. Low-efficiency light sources may comprise low-efficacy lamps, for example, black body radiators, such as incandescent lamps or halogen lamps. Both high efficiency and low-efficiency light sources can be dimmed, but the dimming characteristics of these two types of light sources typically differ. A low-efficiency light source can usually be dimmed to very low light output levels, typically below 1% of the maximum light output. However, a high-efficiency light source cannot be typically dimmed to very low output levels.
Further, high-efficiency and low-efficiency light sources typically provide different color rendering indexes and correlated color temperatures as the light sources are dimmed. A lower color temperature correlates to a color shift towards the red portion of the color spectrum which creates a warmer effect to the human eye. FIG. 1 is a simplified graph showing examples of a correlated color temperature TCFL of a 26-Watt compact fluorescent lamp (i.e., a high-efficiency light source) and a correlated color temperature TINC of a 100-Watt incandescent lamp (i.e., a low-efficiency light source) with respect to the percentage of the maximum lighting intensity to which the lamps is presently illuminated. The color of the light output of a low-efficiency light source (such as an incandescent lamp or a halogen lamp) typically shifts more towards the red portion of the color spectrum when the low-efficiency light source is dimmed to a low light intensity. Conversely, the color of the light output of a high-efficiency light source (such as a compact fluorescent lamp or an LED light source) is normally relatively constant through its dimming range with a slightly blue color shift.
“Color rendering” represents the ability of a light source to reveal the true color of an object. The color rendering index (CRI) is a scale used to evaluate the capability of a lamp to replicate colors accurately as compared to a black body radiator. The greater the CRI, the more closely a lamp source matches the capability of a black body radiator. Typically, low-efficiency light sources, such as incandescent lamps, have high quality color rendering, and thus, have a CRI of one hundred, whereas some high-efficiency light sources, such as fluorescent lamps, have a CRI of eighty as they do not provide as high quality color rendering as compared to low-efficiency light sources.
Generally, many people have grown accustomed to the dimming performance and operation of low-efficiency light sources. As more people begin using high-efficiency light sources—typically to save energy—they are somewhat dissatisfied with the overall performance of the high-efficiency light sources. Thus, it would be desirable to provide a light source that saves energy (like a fluorescent lamp), but provides a broad dimming range and pleasing light color across the dimming range (light an incandescent lamp).